The present invention relates to a circuit element using a magnetic substance, and more particularly, to an irreversible circuit element.
A lumped-element type irreversible circuit element can be configured in a small structure, and has therefore been used as an isolator or a circulator for mobile communication equipment and a terminal thereof early on. The isolator is arranged between a power amplifier and antenna in a transmission stage of communication equipment, used for the purpose of preventing a back flow of unnecessary signals from the antenna of a desired frequency band to a power amplifier and stabilizing impedance of the power amplifier on the load side or the like and the circulator is used for a transmission/reception branching circuit or the like.
FIG. 29 is a see-through perspective view illustrating the inner structure of a conventional lumped-element type isolator (hereinafter, simply referred to as an “isolator”) 100. Furthermore, FIG. 30 is a circuit diagram showing an equivalent circuit of FIG. 29. The equivalent circuit shown in FIG. 30 omits the description of a ferrite plate F1.
As illustrated in FIG. 29, the conventional isolator 100 is made up of three sets of central conductors L1, L2, L3 (each made up of two linear conductors, both ends of which are short-circuited) which are electrically insulated and superimposed so as to intersect each other at an angle of 120 degrees, interposed between a ferrite plate F1 and a ferrite plate F2 (not shown) which is isomorphic to the ferrite plate F1 and permanent magnets (not shown) for magnetizing the ferrite plates F1 and F2 are arranged so as to face each other and sandwich the ferrite plates F1 and F2.
One ends of the respective central conductors L1, L2, L3 are arranged so as to protrude outward from the perimeter of the ferrite plates F1, F2 and those protrusions are connected to signal input/output ports (not shown) and one ends of matching dielectric substrate strips C1, C2, C3 respectively. The other end of each central conductor and the other end of each of the matching dielectric substrate strips C1, C2, C3 are connected to a plane conductor P respectively and the plane conductor P is grounded (not shown). Furthermore, a termination resistor R1 which absorbs a reflected signal is connected to the input/output port of the central conductor L3 and the other end of the termination resistor R1 is grounded (not shown). The central conductors L1, L2, L3 have inductances. Furthermore, the matching dielectric substrate strips C1, C2, C3 together with the central conductors L1, L2, L3 which contact one end thereof and the plane conductor P which contacts the other end thereof each constitute a capacitor (matching capacitor) in an integrated fashion.
In the above described configuration, the isolator 100 displays irreversibility in a certain frequency range by optimizing matching conditions of the matching capacitors or the like, inductances of the central conductors and materials of the ferrite plates F1, F2 or the like. That is, in the frequency range in question, the isolator 100 displays a large attenuation characteristic (isolation) for a signal inputted from the input/output port connected to one end of the central conductor L1 and outputted from the input/output port connected to one end of the central conductor L2, but the isolator 100 has the property of displaying a small attenuation characteristic (or the opposite property thereof) for signals in the direction opposite thereto.
Furthermore, when no termination resistor R1 is provided for the input/output port of the central conductor L3, the isolator 100 becomes a circulator which displays a large attenuation characteristic for a signal inputted from the input/output port connected to one end of the central conductor L1 and outputted from the input/output port connected to one end of the central conductor L2, a signal inputted from the input/output port connected to one end of the central conductor L2 and outputted from the input/output port connected to one end of the central conductor L3 and a signal inputted from the input/output port connected to one end of the central conductor L3 and outputted from the input/output port connected to one end of the central conductor L1, but has the property of displaying a small attenuation characteristic (or the opposite property thereof) for signals in directions opposite thereto.
However, the frequency (operating frequency) bandwidth in which an irreversible circuit element such as a conventional isolator or circulator displays irreversibility is normally a narrow band (e.g., the frequency bandwidth with which it is possible to realize isolation characteristics 20 dB with respect to central frequency 2 GHz is on the order of dozens of MHz).
On the other hand, Non-patent literature 1 discloses a technique for widening the operating frequency bandwidth of an isolator. In this publicly known technique, an inductor and a capacitor are added to the input end of the isolator realizing a characteristic of a fractional bandwidth of 7.7% at central frequency 924 MHz. However, the configuration as described in Non-patent literature 1 with only an inductor and a capacitor added has a limitation in expanding the operating frequency bandwidth from the standpoint of insertion loss or the like and has such a problem that it is not applicable for use in two far-distanced frequency bands.
Furthermore, there is also a technique of providing a plurality of irreversible circuit elements of different operating frequencies and switching between the elements according to the frequency bands used. However, since this technique uses a plurality of irreversible circuit elements, it is difficult to reduce the size of the apparatus. With advanced functionality of portable communication terminal apparatuses in recent years in particular, there is a demand for suppressing the bloating of portable communication terminal apparatuses, and it is difficult to adopt a configuration using a plurality of irreversible circuit elements for such portable communication terminal apparatuses.
Furthermore, Patent literature 1 discloses an irreversible circuit element in which a capacitor for changing the resonance frequency of a resonance circuit is added to the input/output port of each central conductor, an RF switch for disconnecting/connecting this capacitor is provided and the operating frequency is changed through operation of this RF switch. However, this configuration adds capacitors to the input/output ports of the respective central conductors independently, which results in a problem that the number of parts constituting the irreversible circuit element increases.
Non-patent literature 1: “Harmonic Control and Broadening Frequency Bands of Small Isolator” by Hideto Horiguchi, Yoichi Takahashi, Shigeru Takeda, Hitachi Metals Technical Review vol. 17, pp. 58-62, 2001.
Patent literature 1: Japanese Patent Application Laid-Open No. 9-93003